Can i fill a solidify mod while keeping it as a modifier?
You can use a displacement modifier without a texture to widen your cutter object along the normals.
If you don't use a texture, the modifier defaults to 1.0 (white).
I'ts not as convenient as the solify modifier, as the strength doesn't directly translate to blender units. but if your cutter object is in fact just a cube, you can use the dimensions in the transform panel to get a somewhat accurate result.